1. Field of the Invention
The present invention relates to pressurized fuel cell generators, and more particularly relates to an energy dissipater which reduces unwanted heat build-up in the combustion zone of the generator during shut-down of the generator.
2. Background Information
Conventional solid oxide electrolyte fuel cell (SOFC) generators typically include tubular fuel cells arranged in a grouping of rectangular arrays. Each fuel cell has an upper open end and a lower closed end, with its open end extending into a combustion zone. A typical tubular fuel cell has a cylindrical inner air electrode, a layer of electrolyte material covering most of the outer surface of the inner air electrode, and a cylindrical fuel electrode covering most of the outer surface of the electrolyte material. An interconnect material extending along the length of the fuel cell covers the circumferential segment of the outer surface of the air electrode which is not covered by the electrolyte material. An electrically conductive strip covers the outer surface of the interconnect material, and allows electrical connections to be made to an adjacent fuel cell or bus bar. The air electrode may comprise a porous lanthanum-containing material such as lanthanum manganite, while the fuel electrode may comprise a porous nickel-zirconia cermet. The electrolyte, which is positioned between the air and fuel electrodes, typically comprises yttria stabilized zirconia. The interconnect material may comprise lanthanum chromite, while the conductive strip may comprise nickel-zirconia cermet. Examples of such SOFCs are disclosed in U.S. Pat. No. 4,395,468 (Isenberg), U.S. Pat. No. 4,431,715 (Isenberg) and U.S. Pat. No. 4,490,444 (Isenberg). More advanced pressurized SOFC generators are disclosed in U.S. Pat. No. 5,573,867 (Zafred et al.).
During operation of the fuel cell generator, air is provided to an inside air electrode of each tubular cell, and hydrogen-rich fuel is supplied to an outside fuel electrode surface. The fuel and oxidant are utilized electrochemically to produce electrical energy. The depleted air, comprising about 16 percent oxygen, exits the open end of the cell, and the spent fuel of low hydrogen concentration is eventually discharged into a combustion area surrounding the cell open ends.
During normal run conditions, the fuel gas entering the SOFC combustion zone has a low concentration of hydrogen due to the fuel being consumed within the cell stack. In addition, a relatively large amount of oxygen depleted air exits the cells, keeping the air/fuel ratio well beyond stoichiometric in the combustion plenum. This helps to keep the combustion zone temperature at approximately 950xc2x0 C., well within the allowable range for the cells. In addition, the high volumetric flow of air out of each cell may be sufficient to protect the air electrode and open end from any risk of hydrogen reduction.
However, during certain generator stop conditions with the stack in an open circuit condition, that is, loss of grid connection, the air supply may be reduced to a maximum of about 10 percent or less of the normal airflow. The fuel flow to the generator is replaced with a reducing purge flow which serves to protect the fuel electrode from oxidation. This purge flow may cause any stored fuel within the generator to be pushed into the combustion zone where it burns with the available air. There are two primary concerns with this situation. First, the air/fuel ratio is closer to stoichiometric and will result in more combustion and a hotter combustion zone temperature. Second, the reduced air flow leaving each cell may not be sufficient to completely protect the open ends of the cells from hydrogen reduction. Either of these problems have the potential for causing damage to the fuel cells.
Several alternatives have been proposed in the past in an attempt to lessen the severity of this condition. The auxiliary airflow could be increased, thereby reducing the combustion zone temperature, as well as providing added protection for the open ends. This would require larger, more expensive blowers, as well as an uninterruptable power supply large enough to handle their power requirements. The cell open ends presently extend a short distance beyond the upper open end support board, which forms the floor of the combustion zone. Extending the open ends further may move the ends away from the board and reduce the risk of hydrogen reduction, provided that the low airflow still provides air to the board surface so that combustion occurs there and not at the open cell end. However, this approach has the drawback of exposing more of the cell surface area to the flame temperature. Conversely, reducing the cell extension will protect more of the cell surface from the flame, but possibly expose the open ends to more unburned hydrogen. Yet another solution may be to coat the open ends with a material that will prevent reduction of the exposed air electrode.
U.S. Pat. No. 5,023,150 (Takabayashi) taught a fuel cell power generator wherein a resistor is connected by a switching circuit across positive and negative terminals when the generator is shut down. Takabayashi involves clamping a fixed load across the generator terminals. The size of the load is not changed. The load is switched on or off based on the stack voltage. If this is done very rapidly, it has the appearance of controlling the current by changing the effective resistance of the load, without actually changing that resistance. Nonetheless, the actual load resistance remains the same. This type of control is often called time proportioning, because a fixed load is connected across the supply for a portion of the cycle, and disconnected for its balance. Since the Takabayashi invention uses semiconductor switches, it becomes expensive, or unfeasible, when the current is high.
In U.S. Pat. No. 6,025,083, Veyo et al. attempted to solve the above-described problems for non-pressurized SOFC generators by utilizing a fuel dissipater concept, consisting of a fixed resistive load that is switched across the cell stack terminals upon transition to normal or emergency shutdown. The load draws current, which electrochemically consumes the fuel flushed by a nitrogen/hydrogen purge gas mixture used in such situations, thus reducing the combustion zone temperatures and protecting the cells. As the fuel inventory is depleted by the load, the stack voltage drops in response to reduced H2 and CO concentrations and, at some point, a minimum allowable terminal voltage, is reached. The limiting voltage is equal to the nickel oxidation potential at the operating temperature, plus some margin. When this is reached, a voltage sensing circuit disconnects the load by actuating a shunt trip breaker. The voltage sensing and switching circuit can be powered by the stack voltage, making the fuel dissipater xe2x80x9cpassivexe2x80x9d (self-contained). Other dissipater designs may incorporate sensing circuits which are powered by external sources.
The previously described Veyo et al. fuel dissipater design involved a constant resistance value with only two switching functions: on and off. That design consisted of a resistive load bank (in practice, two electric immersion heaters connected in parallel) and a voltage sensing and switching circuit. The heaters were mounted in the steam supply system water tank and were expected to draw about 7 amps/cell. The voltage sensor was an alarm module which actuated a shunt trip breaker when the nickel oxidation voltage (0.62 V nominal) plus a margin (0.05 V) was reached. The electronics were powered by the stack cell terminal voltage using a voltage divider circuit. The expected duration of the dissipation current was about two minutes, until the load was disconnected by the sensed low stack voltage. This worked well for atmospheric pressure SOFC generators, but many recent designs for SOFC generators including hybrid soft/micro-turbine generators, require high pressure operation for greater efficiency. In Veyo, et al., the size of the load was constant and resistance could not be changed in response to a sudden voltage change making it not flexible in changing voltage situations.
However, for an SOFC generator operating at higher pressures (that is, greater than one atmosphere), the conditions and requirements for a fuel dissipater are significantly different. First, the volume of the fuel inventory to be dissipated is significantly higher. In the atmospheric unit, only fuel in open volume is considered. A significant quantity of fuel contained within the porosity of the cell stack insulation boards, usually alumina, is not included, since flow from the porous insulation board to the stack is assumed to be by diffusion only and, therefore, occurs at a slow rate and is considered to be insignificant. However, during shutdown in a pressurized unit, the generator is placed on open circuit and the containment is depressurized. Fuel stored within the board porosity will flow out of the boards due to the depressurization. As a result, the volume of fuel in the boards (approximately 94% porous) must be included in the stored fuel inventory. Second, the fuel flow rate in the pressurized design is controlled primarily by the depressurization rate and is much larger than for the atmospheric design. For representative pressurized generator designs, the nominal cell current can be as high as about 80 amps/cell at 85% fuel consumption, compared to 7 amps/cell for a comparable atmospheric design. For a three-resistor unit, currents in the range of 240 amps may be required by a pressurized design, versus 21 amps for an atmospheric design. Third, the fuel flow rate to the cells is not well known due to various factors which can affect the depress urization rate and the fuel composition.
In the atmospheric generator, the fuel flow rate will be set by the nitrogen/hydrogen mix purge flow rate, which is controlled accurately by orifices in the gas supply line. Also, fuel bypass of the cell stack is not likely to occur at one atmosphere, so that all fuel flow into the stack is assumed to reach the cells without bypassing. In the pressurized design, the fuel flow rate will depend on the exhaust flow rate, the total mass of gas in the test vessel (a function of the temperature gradients inside the test vessel), and the purge flow rates. The fuel purge gas flow rate may be small compared to the fuel flow from depressurization. The exhaust flow may be controlled by a fixed flow resistance (such as, a valve) in the exhaust line. The flow will vary as the system depressurizes, from higher flow at the beginning of the depressurization to lower flow at the end of the depressurization. Furthermore, the hydrogen and carbon monoxide content of the fuel will decline as the fuel is used. The net result is that the fuel consumption at the cells could be significantly higher or lower than the predicted value, depending on how these various factors deviate from the calculated values, making the expected for flow rate and cell voltage difficult to estimate.
As can be seen, a pressurized SOFC generator poses a substantially greater number of difficulties and imponderables during shutdown, to the extent that it is a completely different generator than an atmospheric generator. What is now needed is an advanced energy dissipater design for the new SOFC generators which will operate in a pressurized environment and which can meet changing voltage situations.
The present invention has been developed in view of the foregoing and to address other deficiencies of the prior art.
It is a main object of this invention to provide an improved fuel dissipater that will be effective when used in a pressurized SOFC generator and which can meet changing voltage situations. These and other objects are accomplished by providing a fuel cell generator characterized by and comprising: solid oxide electrolyte fuel cell stacks acting on pressurized hydrogen and carbon monoxide-containing fuel and pressurized oxygen-containing oxidant to provide electrical energy, in which the stacks have positive and negative terminals; a stack energy dissipater which operates on amplitude proportioning of a resistive load, comprising an electrical resistance load, said load comprising an array of at least two cooled, electrically connected resistors controlled by a voltage-sensitive multi-settable point relay, where individual switching contactors allowing for variable resistance loads are disposed between the array and a circuit breaker; where the circuit breaker is in electrical contact with the positive terminal and each of the resistors in the array is in contact with the negative terminal, so that the energy dissipater can draw current, in order to consume hydrogen and carbon monoxide-containing fuel stored within the generator during a transient operation. When a wide range of current is desired, at least three resistors arranged in parallel/series combination is highly preferred. The use of at least three resistors provides the most flexible system. A very useful array contains from three to about seven resistors and FIG. 3 illustrates use of four resistors (resistance elements). The current is dissipated as heat, and the problems associated with the oxidation of hydrogen-rich fuel in the combustion zone of the fuel cell generator are reduced or eliminated.
The invention also includes a method of dissipating energy during shutdown of a fuel cell generator characterized by and comprising: converting pressurized hydrogen and carbon monoxide-containing fuel and pressurized oxygen-containing oxidant to electrical energy in a fuel cell generator; shutting down the fuel cell generator; and drawing current from the fuel cell generator after the generator shuts down thereby to consume at least a portion of the hydrogen and carbon monoxide-containing fuel remaining in the generator and to convert the fuel to oxidized products, thereby to substantially prevent overheating of the generator, wherein the fuel cell generator contains solid oxide electrolyte fuel cell stacks having positive and negative terminals, where a stack energy dissipater which operates on amplitude proportioning of a resistance load is effective to draw current from the fuel cell generator after shutdown by means of an array of at least two electrical resistors providing a load which is electrically connected to the negative terminal at two or more locations, where the electrical resistors are also electrically connected through individual switching contactors and an associated voltage sensitive multi-settable point relay to a circuit breaker, which circuit breaker is electrically connected to the positive terminal at two or more locations, and where the fuel and the oxidant are pressurized to over 151.6 kPa. The term xe2x80x9ckPaxe2x80x9d here means k pascals absolute pressure. In all instances, the term xe2x80x9camplitude proportioning of a resistance loadxe2x80x9d means that the resistance of the load is changed by switching resistors into and out of the circuit, in response to changing fuel cell stack voltage, requiring use of at least 2 resistors. However, as mentioned previously, use of at least 3 resistors provides the most flexible system when a wide range of current is desired.
Thus, this invention requires a network of resistors which maybe switched into and out of the circuit to maintain the stack voltage between minimum and maximum limits, where resistors are individually switched at different voltage levels. The switching of resistors into and out of the circuit in response to changing voltage constitutes control of the stack voltage and current by amplitude modulation of the resistance. After the fuel is oxidized, the energy dissipater is passively and automatically disconnected. The term xe2x80x9cpressurizedxe2x80x9d, as used herein, means operating at a pressure over 1.5 atmospheres (151.6 kPa or 22 psi).
Operation of the xe2x80x9cstack energy dissipaterxe2x80x9d or SED is especially important when depressurizing pressurized hybrid for cell/micro-turbine generator systems. In pressurized systems the quantity of fuel which must be consumed is much greater than in unpressurized systems, and the release rate (and, therefore, the current required to dissipate the fuel) can be much higher. In order to reduce the pressure in the generator, the mass of gas must be reduced by flowing out through the cell stack exhaust. Typically the depressurization rate is very high at the beginning of a shutdown transient and becomes less as the pressure becomes lower. Thus, the flow rate of the fuel vented from the stack is much higher at the beginning of a depressurization transient than it is at the end. It is necessary to draw high current to dissipate the high fuel flow rate early in the depressurization, but this same high current may damage the cell stack later in the depressurization when the flow rate of fuel being vented is lower. Further, the release rate may be difficult to control, or predict, and it may strongly depend on the operating conditions of the generator immediately before depressurization. The depressurization rate may be much higher and the quantity of fuel to be dissipated much larger if the transition to shutdown occurs when the generator is colder, such as during heatup and loading, than if the transition occurs during steady state operation. The fuel composition (and therefore the heating value) will be different if transition to shutdown occurs while the generator is being loaded, than it would be if the transition was from steady state operation. Similarly, the depressurization rate may be much higher if the air flow into the generator is lower, such as during an emergency shutdown situation where the air supply may be bottled air at a rounded flow rate, as opposed to a normal shutdown where the air supply is the gas turbine compressor and the air flow rate is high. The same xe2x80x9cstack energy dissipaterxe2x80x9d or SED may be required to handle all of these various conditions without damaging the cells, and so it is essential for pressurized systems to operate the stack energy dissipator on amplitude proportioning of a resistance load.